The present invention relates to a vehicle having a braking force retaining unit, which is capable of retaining a braking force after releasing the brake pedal such that the braking force continuously acts on the vehicle.
Braking force retaining units are known for continuously retaining braking force on a vehicle after releasing the brake pedal such that the braking force continuously acts on the vehicle. Such a braking force retaining unit facilitates a smooth starting operation of the vehicle on a slope without unintentional backward displacement of the vehicle.
For example, Japanese Patent Publication No.9-202159 discloses a braking force control unit utilizing a traction control system. Such a braking force control unit retains a certain level of braking force by the control of the traction control system until driving force detecting means detects strong driving force (when starting driving force occurs) switched from weak driving force. When driving force is switched to the strong driving force, the retained braking force is released, thereby preventing unintentional backward displacement of the vehicle on an up slope.
The applicant also discloses a brake fluid pressure retaining unit utilizing a brake fluid pressure reduction speed control means in Japanese Patent Application No.10-370249. The brake fluid pressure retaining unit retains braking force until starting driving force is exerted on the vehicle in such a way that reduction speed of brake fluid pressure within a wheel cylinder is less than that of brake pedal load applied by a driver and the brake fluid pressure is gradually reduced. When starting driving force is exerted to start the vehicle, the brake fluid pressure retaining unit releases brake fluid pressure within the wheel cylinder so as to release braking force. Therefore, the driver can start the vehicle on an up slope without unintentional backward displacement. On a down slope, the driver can start the vehicle by releasing or partly loosening the brake pedal.
However, on a steep slope, even if the vehicle is equipped with the above braking force retaining unit, the vehicle often displaces backwards with the displacement force derived from the vehicle""s own weight being greater than the retained braking force. This can happen while the braking force retaining unit is in operation. Especially in the aforementioned braking force control unit, retained braking force may be decreased to a lower value than the initial braking force value that is the braking force retained before starting the release of the brake pedal. In the aforementioned brake fluid pressure retaining unit, retained braking force is not held in a certain value, but is gradually decreased by the brake fluid pressure reduction speed control means. The vehicle tends to displace backwards on a slope if retained braking force is decreased, for the purpose of ensuring a smooth release of the braking force, in advance of occurrence of starting driving force.
With the foregoing drawbacks of the prior art in view, the present invention seeks to provide a vehicle with a braking force retaining unit, which quickly restricts backward displacement of the vehicle when the vehicle starts to displace backward on a slope.
According to the present invention, there is provided a vehicle with a braking force retaining unit, which retains braking force after releasing the brake pedal, wherein the vehicle further comprises means for increasing driving force if displacement of the vehicle is detected while the braking force retaining unit is in operation.
Another object of the invention is to provided a vehicle with a braking force retaining unit, which retains braking force after releasing the brake pedal, wherein driving force of the vehicle is increased if displacement of the vehicle is detected while the braking force retaining unit is in operation.
In such a vehicle with the braking force retaining unit, backward displacement of the vehicle is restricted, even if displacement of the vehicle occurs with the displacement force derived from the vehicle""s own weight being greater than the retained braking force, since driving force is immediately increased when detecting backward displacement (displacement) of the vehicle. Increasing driving force of the vehicle includes the following two cases, these are: (1) driving force occurs from zero and increases, and (2) already existing driving force is further increased.